This invention relates to fiber optic sensors for characterizing a gas-filled gap between a lead-in optical fiber and a lead-out optical fibers. More particularly, this invention relates to a hybrid fiber optic sensor that characterizes the gas-filled gap length by combining the features of a Fabry-Perot interferometer and an intensity-based sensor.
One method of measuring changes in the length of an air gap between a lead-in optical fiber and a lead-out optical fiber is to determine the number of interference fringes arising from the two air-glass interfaces associated with the air gap. Existing interferometric methods, while able to accurately detect changes in air gap length, suffer from an inability to determine whether the air gap length is increasing or decreasing in direction.
Another method for measuring the length of the air gap is by measuring of the intensity of a reflected signal associated with a third interface at the remote end of the lead-out fiber. The amplitude of the reflected signal decreases with an increase in air gap length. Such intensity-based methods, however, are subject to errors arising from intensity changes due to loss mechanisms affecting the lead fibers.